Problems related to well stability are considered the main single source of resource losses during drilling. If the cost of the drilling fluid itself is added, amounts can be substantial.
During oil and gas drilling operations, long sections of argillaceous formations having clay minerals as major constituents, such as shales, mudstones, siltsones, and claystones, often have to be penetrated before reaching the hydrocarbon bearing zones. Various drilling problems, such as bit balling, swelling or sloughing of wellbore, stuck pipe, and dispersion of drilled cuttings, are frequently encountered while drilling such formations. This is especially true when using water-based drilling fluids and can result in tremendous losses of operation time and increases in operation costs. Due to their tendency to become unstable upon contact with water, such argillaceous formations are also commonly referred to as water-sensitive shales.
Upon contact of a water-based mud with shales, water adsorption occurs immediately. This causes clays to hydrate and swell resulting in stress and/or volume increases. Stress increases can induce brittle or tensile failure of the formations, leading to sloughing, cave in and stuck pipe. Volume increases, on the other hand, reduce the mechanical strength of shales and cause swelling of wellbore, disintegration of cuttings in drilling fluid, and balling up of drilling tools. In order to prevent water adsorption and clay hydration from occurring, it is believed that oil-based drilling fluids are the most effective for this purpose.
The inhibitive action of oil-based drilling fluids is based on the emulsification of brine in oil, which acts as a semi-permeable barrier that materially separates the water molecules from being in direct contact with the water-sensitive shales. However, there are many restrictions to the use of oil-based drilling fluids in view of their detrimental effect on the environment.
On the other hand, it is well-known that the correct choice of the fluid to be used during the drilling of an oil well is directly responsible for the economical and technical success of the operation. In many instances, the interaction between the fluid and the rock being drilled causes undesirable effects with costs which are increased beyond all expected levels.
Several hypotheses and theories were advanced to try to explain the mechanisms which govern the instability of an oil well, one of the most widely known being the physico-chemical changes caused by the contact of shales with the drilling fluid. In order to minimize the drawbacks caused by these changes, various additives have been developed, which are compounded to the base fluid according to the nature of the rock to be drilled.
The contact of shales with aqueous drilling fluids may cause swelling of shales, this being attributable to at least two mechanisms, crystalline and osmotic. Crystalline swelling or surface hydration results from the adsorption of mono-molecular layers of water onto the shale surfaces. Osmotic swelling occurs if the cation concentration on the surfaces of the shale is greater than that in the surrounding fluid. Ion hydration and water adsorption causes an increase in the hydrodynamic volume. For swelling to occur, the shale must interact with the water taking and/or sharing hydroxyl groups of the water with it. Normally shale inhibition is achieved by adding divalent cations or potassium ions through base exchange or by adding encapsulating and bridging polymers to the water based fluids or by the use of oil based fluids.
In the available drilling fluid systems, the evaluation and classification of the adequacy of the drilling fluid to the rock to be drilled (or being drilled) is of paramount importance for the success of the drilling operation in the well.
The presently available methods for the qualitative and quantitative evaluation of the adequacy and quality of the drilling fluid relate mainly to changes and alterations in the weight of the rock under test in response to the action of a drilling fluid.
Thus, the Slake Durability Test compares the weight of a dried sample before and after immersion in the test fluid. The weight of the dried sample is the weight after drying in an oven for 24 hours. It is evaluated that the lesser the weight loss from the sample, the more adequate is the fluid, since less dispersion was caused in the solution.
The Hot Rolling Test consists of a similar procedure, however the sample is rolled in the fluid at a temperature which is higher than ambient.
The Linear Swelling Test is based on the assumption that some clay minerals such as montmorillonites are able to absorb huge amounts of water when exposed to an aqueous fluid, this causing the swelling of the shale. Thus, this test measures, for a cylindrical sample of rock, the expansion in the axial direction as a function of the immersion time of the sample in the test solution. The expansion or contraction of the sample is measured as a percentage relative to the original length of the test sample. In spite of the fact that doubts persist on the interpretation of the obtained results, it is believed that the best drilling fluid is that one which causes the least expansion of the rock sample. Generally various fluid compositions are tested, in order to find that one which will cause the slightest possible expansion and preferably a certain degree of contraction in the sample.
Tests for the measurement of the swelling of a clay-rich shale such as a shale when contacted with an aqueous solution have been suggested by the International Society of Rock Mechanics (ISRM). The Swelling Pressure Test may be found for example in the patents by Ronald Steiger, see U.S. Pat. No. 5,275,063. Referring to the controversial aspect of the swelling of clay minerals, the fluid which will cause the least swelling pressure on the shale would be the most appropriate, since it would cause the minor possible changes in the shale. In some of the tests the swelling pressure is calculated from the measurement of the sample deformation.
In qualitative tests, a visual and tactile inspection of the sample is effected after immersion of the rock in the test fluid.
The X-Ray Diffraction (XRD) method is used as a tool for evaluating the mineralogy of a sample which has been previously dried and powdered.
The patent literature is abundant in references on the subject of the present application.
U.S. Pat. No. 4,963,273 teaches a modified liquid phase drilling fluid having desirable properties of shale swelling inhibition, lubrication, and high temperature performance. Shale inhibition is the ability of a process to retard the hydration of shales whereby they remain intact and basically in their original size, shape and volume. The fluid comprises a liquid phase containing a water phase and a water-soluble component selected from polyhydric alcohols, glycol, glycol ethers, etc., ethylene oxide-propylene oxide copolymers (EO-PO); a viscosifier; and a filtration control agent. It is alleged that the circulation of the drilling fluid into, through and out of said subterranean well will cause that the drilling fluid contacts formation particulate matter in the bore of the well. The water-soluble component is considered as providing lubricity and shale inhibiting properties comparable to those of an oil-based drilling fluid without the adverse effects thereof.
U.S. Pat. No. 5,205,164 teaches a method for determining drilling fluid density for stabilizing a wellbore including obtaining shale cuttings from a wellbore. Shale cuttings are obtained from a wellbore and their index properties, including but not limited to surface area, is measured; mean effective stress around the wellbore is calculated using the geostatic overburden in situ stress, the field pore pressure, and the total stress around the wellbore; the in situ shale strength is determined using a correlation between surface area, mean effective stress and shale strength; and drilling fluid density is determined using the shale strength. It is assumed that shale strength correlations can be used at a rig site to predict stable wellbore conditions. Shale cuttings are not preserved from dehydration.
U.S. Pat. No. 5,645,458 teaches a water-based drilling fluid that comprises a water-miscible glycol, with a molecular weight of less than about 200, at a concentration of from 30 to 70% by weight of the aqueous phase of the drilling fluid; an organic cationic material or organic salt of potassium such as choline hydroxide, choline chloride, choline carbonate in a range from 3% by weight up to saturation, based on the aqueous phase of the drilling fluid; a filtration control agent for lowering fluid loss of the drilling fluid; a viscosifier for suspension of solids and weighting material in the drilling fluid; and water. It is alleged that the inhibitive effects of the described fluids on water-sensitive shales have been evaluated by conducting swelling tests, dispersion tests and triaxial tests conducted under simulated downhole conditions. The aim of the suggested composition is to replace the water with a polar fluid which can compete with water for adsorption but will not severely weaken the mechanical strength of argillaceous rocks when adsorbed.
On examining the information sources on the subject it is clear that on the one hand simple tests such as the Slake Durability Test and the Hot Rolling Test are available for determining the interaction of fluids and clay-rich rocks while on the other hand XRD is a sophisticated tool which is able to depict the mineralogy of a sample which has been dried and powdered, so that its hydration state in the downhole condition and after contact with a test fluid cannot be established.
It is then clear that none of these tests, either by themselves or combined in any way, are able to assess the rock-fluid interaction on a truly preserved, downhole condition rock sample.
Thus, in spite of all the progress achieved in this field, the open literature invariably discloses processes focusing on shales which have been tested in a dehydrated condition, this leading to altered results which in turn affect the proposed improvements in drilling fluid compositions. This, as expected, leads to losses due to inadequate drilling fluid compositions. These observations are in line with the contents of the co-pending application "A Method for the Evaluation of Shale Reactivity", also in the name of Mr. Helio M. R. dos Santos, inventor of the present application.
On the other hand, the characterization of a shaly rock, that is, the determination of the presence or not of swelling-inducing minerals, would certainly be helpful in devising the most adequate drilling fluid for a specific rock composition.
Thermo-Gravimetric Analysis or TGA is a well-known technique based on the principle that mass losses from a certain sample at a defined temperature correspond to the presence of specific substances present in the sample under test.
On a TGA experiment, each mineral has its own signature, that is, it is possible to at least qualitatively identify substances in a sample, for example a shale. Thus by associating the shale signature and that from the individual minerals, it is possible to determine the composition of a shaly rock.
Santos, H. and da Fontoura, S. A. B., in SPE paper n.sup.o 38644 presented at the 1997 SPE Annual Technical Conference and Exhibition, San Antonio, Tex., Oct. 5-8, 1997 "Concepts and Misconceptions of Mud Selection Criteria: How to Minimize Borehole Stability Problems?" discuss the mechanisms of swelling and selection of drilling fluids. It is stated that shale swelling is strongly related to both the amount and distribution of water within the rock. Water present in a shale may be free, interlayer, bound or crystalline water. From their experimental observations on shaly rocks authors concluded that efforts should focus not on the fact that shale swells, but rather, if the downhole shale swells. This is because it could be seen that shale only swells if it loses some of its original water, disturbing its equilibrium. On the contrary, in downhole condition, the shale is in physical and chemical equilibrium, the reaction being minimal when in contact with water, this being shown by experiments with truly preserved shales, that is, shales which have not dehydrated and therefore preserve the original, downhole hydration condition. Therefore both smectite content and period of air-exposure time will influence the swelling of a shale sample. Authors recommend careful handling procedure in the laboratory in order to preserve the downhole hydration condition of the shales.
It seems clear from the state-of-the-art technique that measurements on the rock-fluid interaction are effected on non-preserved shales, with the ensuing erroneous results. On the contrary, and according to a patentably distinguishing feature of the present invention, the rock-fluid interaction as measured by the modification of the TGA graph is effected on a sample free of contact with the drilling fluid or mud while are kept the original, downhole hydration conditions of the rock. The present method thus makes possible to effect measurements which reproduced the downhole conditions of the tested rock sample.
Therefore, the technique is in need of a method for exactly determining the rock composition and its water content under downhole condition, and thus evaluating in an accurate and precise way the changes undergone by a shaly sample when in contact with any drilling fluid. Such a method, which employs TGA for determining the rock composition is described and claimed in the present application.